Layered inductor

ABSTRACT

A layered inductor  10  is manufactured by layering “silver-based conductive layers” and “ferrite-based magnetic layers” and simultaneously firing these layers. The conductive layers are via-connected to form a helical coil  30.  A shape of a cross sectional surface of the conductive layer, cut by a plane perpendicular to a longitudinal direction of each of the conductive layers is a substantial trapezoid shape, having an upper base and a lower base. A base angle θ of the trapezoid shape at both ends of the lower base is equal to or greater than 50° and is smaller than or equal to 80°.

FIELD OF THE INVENTION

The present invention relates to a layered inductor in whichsilver-based conductive layers and ferrite-based magnetic layers arelayered, and simultaneously fired/sintered.

BACK GROUND OF THE INVENTION

Conventionally, a compact power inductor (chip inductor) has been usedfor realizing functions such as suppressing noise, rectification, andsmoothing signals, for example, in a power supply circuit forsemiconductors, a power circuit for a DC-DC convertor, and so on. Alarge inductance and a low resistance are required for the compact powerinductor.

One of the compact power inductors is a layered inductor (multi-layeredinductor). As shown in FIG. 36 showing a perspective view of the layeredinductor 110, the layered inductor 110 typically comprises a magneticbody 111 and a coil 112 buried in the magnetic body 111.

The coil 112 comprises conductive layers (conductive thin plates) 112A,each of which is formed so as to have a predetermined shape, andconductive connecting portions 112B in via holes (VIA), each of whichelectrically via-connects (wiring-connects between layers) between theconductive layers adjacent to each other in a direction of layering (ina vertical direction), and is formed to have a helical shape. This typeof layered inductor 110 is manufactured, for example, by printing andlayering method, tape layering method, and the like.

In order to lower a resistance in such a layered inductor 110, it isnecessary to increase a cross sectional area of the coil 112. However,as shown in FIG. 37, according to the printing and layering method, thetape layering method, or the like, magnetic-layers-before-fired 111 aare firstly prepared and a conductive-layer-before-fired 112 a is formedon each of the magnetic layers 111 a. Subsequently, as shown in FIG. 38,they are layered/laminated to form a layered body, and thereafter, thelayered body is fired (burnt/sintered). Accordingly, as shown in FIG.38, thickening the conductive layer 112 a in order to increase the crosssectional area of the coil 112 leads to a large difference between athickness X1 of a portion where the conductive layers 112 a are formedand a thickness X2 of a portion where no conductive layer 112 a isformed.

As a result, when the layered body is fired, a gap (a crack) may begenerated/occurred between the magnetic layers 111 a, 111 a adjacent toeach other in the direction of layering, and in some cases, a problemthat a structural defect may occur arises, e.g., one of the magneticlayers 111 a is delaminated (or removed) from another layer 111 aadjacent to the one (i.e., a delamination occurs). Further, there may beanother problem that the inductor does not have desired electricalcharacteristics, such as a great decrease in the inductance, due to thestructural defect.

In order to cope with the problems, it is proposed that gaps around theconductive layers 112 a are formed beforehand (refer to, for example,Patent document 1 and Patent document 2). However, it is necessary toincrease a cross sectional area of the gap as the cross sectional areaof the conductive layer 112 a increases. As a result, it is stilldifficult to decrease the resistance of the inductor without anexcessive decrease of the inductance of the inductor, even by theproposed technique.

[Prior Art Document]

[Patent Document]

[Patent Document 1] Japanese Patent No. 2987176

[Patent Document 2] Japanese Patent No. 4020131

SUMMARY OF THE INVENTION

Meanwhile, a gel cast method (gel-casting method) is a method where aceramic slurry in which ceramic powders and gelatinizing agents aredispersed in the dispersion media (solvent) is poured into a mold, andthereafter, the slurry is fixed/hardened according to an urethanereaction so as to obtain a formed-compact-before-fired (pre-sinteredbody, pre-fired body). According to the gel cast method, a formedcompact having a desired shape to be formed by the mold can easily beobtained, because an amount of shrinkage of the slurry when the slurryis dried (or when the solvent vaporizes) is small compared with thegeneral forming method.

In view of the above, the applicant is trying to manufacture “a ceramicgreen sheet in which a conductive layer is buried into a magnetic layer(e.g., ferrite layer)” which has a uniform thickness, by using the gelcast method. Specifically, a conductive layer having a predeterminedshape is firstly formed on an upper surface of a lower mold by aprinting method. Subsequently, an upper mold is placed on the lower moldto form/provide a space having a constant height tohold/store/accommodate the conductive layer. Thereafter, “a ceramicslurry, in which ferrite powders as ceramic powders and gelatinizingagents are dispersed in dispersion media” is poured into the space to befixed/hardened in order to form a ferrite-sheet-before-fired in whichthe conductive layer is buried. As a result, the sheet-before-fired,having the uniform thickness, in which the conductive layer has anincreased thickness, can be manufactured/produced.

This sheet has the uniform thickness. Accordingly, in a layered bodyobtained by layering the sheets, “a thickness of a portion where theconductive layers are formed” and “a thickness of a portion where noconductive layer is formed” are equal to each other. This allows thethickness of the conductive layer to be increased without increasing adistance (a pitch) between conductive layers adjacent to each other in adirection of layering (in the other words, without decreasing theinductance), since the problem such as the delamination hardly occurs.As a result, it is possible to decrease the resistance of the layeredinductor without decreasing the inductance of the layered inductor.

In the compact/body formed according to the gel cast method, the ceramicpowders are captured in polymer networks, and the amount of shrinkageduring drying is therefore small, however, a density of the compact/bodybecomes small compared to the compact/body formed according to theprinting-layering method, and the tape layering method, etc. This causesa large difference in an amount of shrinkage between the conductivelayers formed by the printing method and the magnetic layers formed bythe gel cast method, while they are fired. As a result, while firing,the structural defect, such as “gaps extending in a direction parallelto layers (hereinafter, referred to as “a side direction gap” or “a sidedirection crack”) may occur, or the desired electrical characteristicsmay not be obtained. Particularly, if a ratio of a volume of theconductive layers to a volume of the inductor is made larger in order toreduce the resistance of the coil, the structural defect may occurprominently. Accordingly, it turned out that there arises a problem thatthe inductance decreases extremely.

One of the objects of the present invention is to provide a layeredinductor (multi-layered inductor), manufactured using the gel castmethod, which has a structure that can avoid the problems describedabove.

The layered inductor according to the present invention to achieve theobject is a layered body in which “silver-basedconductive-layers-before-fired (conductive layers that have not beenfired)” and “ferrite-based magnetic-layers-before-fired (magnetic layersthat have not been fired)” are layered and thereafter simultaneouslyfired, and in which the conductive-layers-after-fired (fired conductivelayers) are via-connected so as to form a helical coil in themagnetic-layers-after-fired (fired magnetic layers).

Further, the layered inductor of the present invention is characterizedin that,

a shape of a cross sectional surface of each of the conductive layerscut/taken by a plane perpendicular to a longitudinal direction of eachof the conductive layers is a substantial trapezoid shape having anupper base and a lower base; and

a base angle θ of the trapezoid shape at both end portions of the lowerbase is equal to or greater than 50° and is smaller than or equal to 80°(50°≦θ≦80°.

According to the features above, a gap (the side direction crack)extending along the direction parallel to the plane of the layer hardlyoccurs, because “the shape of the cross sectional surface of each of theconductive layers cut by the plane perpendicular to the longitudinaldirection of each of the conductive layers (i.e., a shape of the crosssectional surface of the coil)” is substantially trapezoid, and the baseangle θ of the trapezoid shape is equal to or greater than 50° and issmaller than or equal to 80°, compared with an inductor in which a shapeof a cross sectional surface of the coil is a semi circular shape (or anarc shape). Accordingly, the above delamination and the like do notoccur. As a result, the layered inductors having stable electricalcharacteristics can be provided, according to the present invention.

In this case, it is preferable that the magnetic layers have “a gap/gapsextending so as to have a component along a direction of layering of thelayered body (i.e., a component along a direction perpendicular to eachof the planes of the layers) and so as to connect between two of theconductive layers adjacent to each other in the direction of layering”.“The gap extending so as to have the component along the direction oflayering” is referred to as “a vertical direction gap” for convenience.

In the present invention, the shape of the cross sectional surface ofthe coil is substantially trapezoid, and thus, stress generating duringfiring process (especially when the body is cooled) concentrates on “theend portions of the lower base” and “the end portions of the upperbase”. Accordingly, the vertical direction gaps can be positively formedfrom those end portions as origination points.

The vertical direction gap does not induce (or cause) the delamination,unlike “the side direction gap” extending in the direction parallel toplanes of the layers. In addition, the vertical direction gap canrelease a great internal stress applied to the ferrite-based magneticlayers. Generally, the great internal stress applied to the magneticlayers made of/from ferrite causes a great change in the inductance.Accordingly, having such vertical direction gaps occur positively canprovide the inductor whose inductance is in proximity to (close to) adesired/targeted value.

Furthermore, in this case, it is preferable that each of the gaps, in across sectional view of the conductive layers and the magnetic layerscut/taken by the plane perpendicular to a longitudinal direction of theconductive layers, extend “downwardly so as to have the component alongthe direction of layering” from “a surface of each of the conductivelayers within ±30 μm along the surface of each of the conductive layersfrom one of end portions of the lower base of each of the conductivelayers”, and extend “upwardly so as to have the component along thedirection of layering” from “a surface of each of the conductive layerswithin ±30 μm along the surface of each of the conductive layers fromone of end portions of the upper base of each of the conductive layers”.

In this case, it is also preferable that one of the vertical directiongaps extend downwardly from an end portion located at an outercircumference side of the coil among both end portions of the lower baseof specific one of the conductive layers, another one of the verticaldirection gaps extend upwardly from an end portion located at the outercircumference side of the coil among both end portions of the upper baseof another one of the conductive layers which is adjacent to and beneaththe specific one of the conductive layers, and those two vertical gapsbe connected/communicated to each other.

Furthermore, one aspect of the layered inductor having any one of thefeatures described above according to the present invention is aninductor wherein,

the fired conductive layers have a great number of holes/pores and aratio of a total area of the pores to an area of the conductive layer ina cross sectional view of the conductive layers, cut/taken by a planeperpendicular to the longitudinal direction of the conductive layers, isequal to or greater than 2% and is smaller than or equal to 30%, and

a ratio D/t1 of an average diameter D of the pores to the thickness t1of each of the fired conductive layers is equal to or greater than 0.01and is smaller than or equal to 0.20.

If the area ratio of the pores is smaller than 2%, a hardness of thecoil (the fired conductive layers) is so high that a stress can not beconcentrated on the end portions of “the upper base and the lower base”of the coil, and the large side direction gaps may therefore occur inthe magnetic layers, and thereby, the inductance is not stable. On theother hand, if the area ratio of the pores is greater than 30%, thecross-sectional area of the coil is excessively small, and theresistance of the coil therefore becomes excessively large.

In addition, the stress can be concentrate more easily on the endportions of “the upper base and the lower base” of the coil, when “thepores each having relatively small diameter” which satisfy the describedcondition that the ratio D/t1 is equal to or greater than 0.01 and issmaller than or equal to 0.20 are dispersed in the coil.

In view of the above, the layered inductor having the inductance inproximity to the desired and targeted value and having the lowresistance coil can be provided by means of the above feature.

In this case, it is preferable that a portion of the fired magneticlayer, the portion existing between two of the conductive layers thatare adjacent to each other in the direction of layering, have a relativedensity which is equal to or greater than 84% and is smaller than orequal to 92%, wherein the relative density is 100% when it is assumedthat there is no pore in the magnetic layers.

If the relative density of the specific portion is smaller than 84%, ahygroscopicity of the magnetic layer is so high that the reliability ofthe layered inductor may become low. On the other hand, if the relativedensity of the specific portion is greater than 92%, uncontrollable gaps(side direction gaps) occur in the magnetic layers. Accordingly, it ispossible to provide the layered inductor having the inductance inproximity to the targeted value and high reliability, by having theabove feature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a layered inductor according to anembodiment of the present invention;

FIG. 2 is a vertical cross-sectional view of the layered inductor shownin FIG. 1;

FIG. 3 is a perspective view of one of ceramic green sheets whichconstitute the layered inductor shown in FIG. 1;

FIG. 4 is a cross-sectional view of one of the ceramic green sheetswhich constitute the layered inductor shown in FIG. 1;

FIG. 5 is a perspective view of another one of the ceramic green sheetswhich constitute the layered inductor shown in FIG. 1;

FIG. 6 is a perspective view of another one of the ceramic green sheetswhich constitute the layered inductor shown in FIG. 1;

FIG. 7 is a perspective view of another one of the ceramic green sheetswhich constitute the layered inductor shown in FIG. 1;

FIG. 8 is a perspective view of another one of the ceramic green sheetswhich constitute the layered inductor shown in FIG. 1;

FIG. 9 is a perspective view of another one of the ceramic green sheetswhich constitute the layered inductor shown in FIG. 1;

FIG. 10 is a perspective view of another one of the ceramic green sheetswhich constitute the layered inductor shown in FIG. 1;

FIG. 11 is a perspective view of another one of the ceramic green sheetswhich constitute the layered inductor shown in FIG. 1;

FIG. 12 is a view showing processes to manufacture the layered inductorshown in FIG. 1;

FIG. 13 is a view showing processes to manufacture the layered inductorshown in FIG. 1;

FIG. 14 is a view showing processes to manufacture the layered inductorshown in FIG. 1;

FIG. 15 is a view showing processes to manufacture the layered inductorshown in FIG. 1;

FIG. 16 is a perspective view of a ceramic green sheet which constitutesthe ceramic green sheet shown in FIG. 3;

FIG. 17 is a perspective view of the other ceramic green sheet whichconstitutes the ceramic green sheet shown in FIG. 3;

FIG. 18 is a view showing processes to manufacture the layered inductorshown in FIG. 1;

FIG. 19 is a view showing processes to manufacture the layered inductorshown in FIG. 1;

FIG. 20 is a view showing processes to manufacture the layered inductorshown in FIG. 1;

FIG. 21 is a view showing processes to manufacture the layered inductorshown in FIG. 1;

FIG. 22 is a graph showing temperature in a firing process tomanufacture the layered inductor shown in FIG. 1;

FIG. 23 is a vertical cross-sectional view of conductive layers (coilportion) according to an example of the present invention;

FIG. 24 is a vertical cross-sectional view of conductive layers (coilportion), each having another shape, according to an example of thepresent invention;

FIG. 25 is a vertical cross-sectional view of conductive layers (coilportion), each having yet another shape, according to an example of thepresent invention;

FIG. 26 is a vertical cross-sectional view of conductive layers (coilportion), each having still another shape, according to an example ofthe present invention;

FIG. 27 is a vertical cross-sectional view of conductive layers (coilportion), each having another shape, according to an comparativeexample;

FIG. 28 is a partial vertical cross-sectional view of a conductive layer(coil portion), according to an example of the present invention;

FIG. 29 is a graph showing a relationship between “a ratio of thicknessof the coil portion to thickness of a magnetic layer between coils” and“a percent defective”;

FIG. 30 is a photograph showing a vertical cross section of a layeredinductor according to the example of the present invention;

FIG. 31 is a magnified photograph of the vertical cross section of aportion around the coil portion shown in FIG. 30;

FIG. 32 is a photograph showing a vertical cross section of a layeredinductor according to the comparative example;

FIG. 33 is a photograph showing a vertical cross section of a layeredinductor according to another example of the present invention;

FIG. 34 is a magnified photograph of the vertical cross section of aportion around the coil portion shown in FIG. 33;

FIG. 35 is a photograph showing a vertical cross section of a layeredinductor according to another example of the present invention;

FIG. 36 is a perspective view of a conventional layered inductor;

FIG. 37 is a cross-sectional view of a sheet for the layered inductor,the sheet being manufactured according to a conventional printing andlayering method and the like; and

FIG. 38 is a cross-sectional view of a layered body obtained by layeringthe sheets shown in FIG. 37.

DESCRIPTION OF THE EMBODIMENTS CARRYING OUT THE INVENTION

Next will be described layered inductors (layered type inductors,multi-layered inductors) according to embodiments of the presentinvention with reference to the drawings.

<Structure of the Layered Inductor>

FIG. 1 is a perspective view of a layered inductor 10 according to anembodiment of the present invention. FIG. 2 is a verticalcross-sectional view of the layered inductor 10, wherein the inductor iscut by a plane along a line 1-1 of FIG. 1. A shape of the layeredinductor 10 is a rectangular parallelepiped, having a depth, a width,and a height, each of which is about a few millimeters. The layeredinductor 10 comprises a magnetic body portion 20 containing ferrite as amagnetic substance (i.e., ferrite-based magnetic body 20) and coilportion 30 containing silver (Ag) as a conductive material (i.e.,silver-based coil portion 30). The magnetic body portion 20 comprises aplurality of magnetic layers integrated/united by being fired. The coilportion 30 comprises a plurality of conductive layers integrated/unitedby being fired.

The coil portion (the conductive portion) 30 is buried in the magneticbody portion 20 in such a manner that the coil portion 30 has a helicalshape. An outer configuration and an inner configuration of the coilportion 30 are both roughly rectangular in plan view. The coil portion30 is strip-shaped, has a substantially constant width, and is formedof/from “conductive layers containing silver (Ag) as a main component(silver-based-conductive-films)”. The width L1 of the coil portion 30will be described later. A total number of turns of the coil portion 30is 7.25. It should be noted that the total number of turns (turn number)of the coil portion 30 may appropriately be changed according to adesign. For example, the total number of turns of the coil portion 30may be equal to or more than 5 and be less than or equal to 9.

The layered inductor 10 is manufactured by layering/laminating andpressure bonding a plurality (eight from a first layer 41 to an eighthlayer 48 in the present example) of ceramic green sheets shown in FIGS.3-11, layering and pressure bonding another ceramic green sheets whichare not shown (an uppermost ceramic green sheet and a lowermost ceramicgreen sheet) onto each of an upper surface of and a lower surface of thepressure bonded sheets to form “a layered-body-before-fired”, andthereafter, firing “the layered-body-before-fired” simultaneously.Hereinafter, each of the ceramic green sheets is simply referred to as“a sheet”. Each of the sheets 41-48 has a uniform thickness.

FIG. 3 is a perspective view of the first layer sheet 41. (a) and (b) ofFIG. 4 show cross sectional views of the sheet 41, cut by a plane alonga line 2-2 and a plane along a line 3-3 of FIG. 3, respectively. A shapeof the sheet 41 in plan view is roughly rectangular.

The sheet 41 is composed of “a magnetic-layer-before-fired 21” whichwill constitute a part of the magnetic body portion 20 shown in FIGS. 1and 2, and “a conductive-layer-before-fired 31” which will constitutethe coil portion 30 shown in FIGS. 1 and 2.

The magnetic layer 21 is a thin plate obtained by pouring ceramicslurry, in which “ferrite powders as ceramic powders” and “gelatinizingagents” are dispersed in dispersion media, into a mold having apredetermined shape, and thereafter, drying and fixing/hardening theslurry. The magnetic layer 21 has a uniform thickness.

The conductive layer 31 is a thick film obtained by forming/shapingconductive paste comprising silver (Ag) as conductive powders, resinsdescribed later, and an organic solvent described later, on a lower molddescribed later according to a printing method, and thereafter, bydrying and fixing the paste.

The conductive layer 31 is composed of a main conductive layer 31 a anda via-connection portion 31 b.

The main conductive layer 31 a is a portion for forming a winding wireportion of the coil portion 30, and is formed so as to have a shapewhich constitutes a lowermost winding portion of the coil portion 30after firing. That is, the main conductive layer 31 a is, in plan view,has a shape, “which is strip-shaped and has the constant width, andwhose outer circumference is roughly rectangular (or roughly square)”.The thickness of the main conductive layer 31 a is smaller than thethickness of the magnetic layer 21. A lower surface of the mainconductive layer 31 a exists on the same plane as a lower surface of themagnetic layer 21. That is, the lower surface of the main conductivelayer 31 a is exposed at the lower surface of the magnetic layer 21. Across sectional surface of the main conductive layer 31 a, cut by aplane perpendicular to a longitudinal direction of the main conductivelayer 31 a (that is, cut by a plane along a width direction of the mainconductive layer 31 a) has a trapezoid shape substantially, as shown in(a) and (b) of FIG. 4. The shape of the main conductive layer 31 a afterfired will be described later in detail.

The via-connection portion (wiring-connection-portion between layers) 31b is a portion which electrically connects between the main conductivelayer 31 a and a main conductive layer 32 a included in “the secondlayer sheet 42 shown in FIG. 5” which is layered on the first layersheet 41. The via-connection portion 31 b is connected to the uppersurface of the main conductive layer 31 a at the end of the mainconductive layer 31 a, and is exposed at the upper surface of the sheet41. A shape of the via-connection portion in plan view is roughly squarewhose side has a length equal to the width of the main conductive layer31 a.

As described above, the sheet 41 has a constant thickness at anyportions. That is, the upper surface and the lower surface of the sheet41 form flat planes parallel to each other. It is therefore said thatthe sheet 41 is a sheet, which is thin plate-like and has a uniformthickness, comprising “the magnetic-layer-before-fired 21” which willform the magnetic body portion 20 and “the conductive-layer-before-fired31” which is buried in the magnetic-layer-before-fired 21.

The second layer sheet 42 to the eighth layer sheet 48, as shown inFIGS. 5 to 11, respectively, are different from the first layer sheet 41only in that the sheets comprise conductive layers 32-38, respectively,each of which has a shape different from the shape of the conductivelayer 31 of the first layer sheet 41 in plan view.

The main conductive layer 32 a of the second layer sheet 42 iselectrically connected to the main conductive layer 33 a of the thirdlayer sheet 43 through the via-connection portion 32 b. In the samemanner, the main conductive layer 33 a of the third layer sheet 43 iselectrically connected to the main conductive layer 34 a of the fourthlayer sheet 44 through the via-connection portion 33 b. The mainconductive layer 34 a of the fourth layer sheet 44 is electricallyconnected to the main conductive layer 35 a of the fifth layer sheet 45through the via-connection portion 34 b. The main conductive layer 35 aof the fifth layer sheet 45 is electrically connected to the mainconductive layer 36 a of the sixth layer sheet 46 through thevia-connection portion 35 b. The main conductive layer 36 a of the sixthlayer sheet 46 is electrically connected to the main conductive layer 37a of the seventh layer sheet 47 through the via-connection portion 36 b.The main conductive layer 37 a of the seventh layer sheet 47 iselectrically connected to the main conductive layer 38 a of the eighthlayer sheet 48 through the via-connection portion 37 b. In this way, themain conductive layers of the sheets are via-connected so as to form ahelical coil.

It should be noted that the via-connection portions 32 b-38 b of thesecond layer sheet 42-eighth layer sheet 48 are formed at positions inthe sheets, the positions being different from one another in plan view.This allows each of sides of the coil portion 30 to have an equal numberof via-connection portions to one another in plan view as much aspossible. Notably, it is not necessary to form the via-connectionportion 38 b, because no conductive layer is formed on the eighth layersheet 48 shown in FIG. 11. However, in the present example, a dummyvia-connection portion (hereinafter, referred to as “a dummy via”) 38 bis formed so that each of sides of the coil portion 30 can have an equalnumber of via-connection portions to one another as much as possible inplan view of the coil portion 30. It is preferable that the number ofthe dummy via be adjusted depending on the number of turn of the coilportion 30. For example, if the number of turn is small, the number ofthe dummy via may be large in order to adjust a balance of the coil. Tothe contrary, if the number of turn is large, there may be a case whereno dummy via is necessary.

<Method for Manufacturing>

Next will be described a method for manufacturing the layered inductors10.

1. Preparation (Forming) for Material of the Magnetic Layer.

First, a method for preparing the material of the magnetic layer isdescribed.

1.1: Preparation for Ferrite Powders 1.1.1: Weighing, Mixing, and Drying

Each of Fe₂O₃ (grain diameter 0.5 μm), ZnO (grain diameter 0.3 μm), NiO(grain diameter 1 μm), CuO (grain diameter 2 μm), and MnO₂ (graindiameter 2 μm) is weighed. The weighed raw material powders are put intoa POLYPOT together with zirconia balls and an ion-exchange water, andthereafter, they are wet mixed by a ball milling method for 5 hours toobtain a slurry. The slurry is dried by a drying oven, and then issieved to obtain powders.

1.1.2: Preliminary-Firing, Milling, and Drying

Subsequently, thus obtained powders are heat treated (or preliminarilyfired) for 2 hours at 760° C. (that is, at heat treatment temperaturefor preparing ferrite powders). It is preferable that this preliminaryfiring temperature be a temperature lower than a temperature at whichferrite haploidization occurs by 50 to 200° C. For example, thepreliminary firing temperature may be an appropriate temperature within600 to 800° C. When the preliminary firing is performed, a rate oftemperature increase and a rate of temperature decrease are 200° C./h.Subsequently, the heat treated powders are put into a POLYPOT togetherwith zirconia balls and an ion-exchange water, and thereafter, wetmilled by the ball milling method for 60 hours to obtain a slurry. Thetime duration for the ball milling may be an appropriate duration within10 to 80 hours. The thus obtained slurry is dried by the drying oven,and then is sieved to obtain ferrite powders.

1.2: Preparation (Conditioning, Mixing) of a Ferrite Slurry

The obtained ferrite powders, solvent•dispersion media (glutaric aciddimethyl, triacetin), and a dispersing agent (carboxylic copolymer, forexample, MALIALIM (trade name)) are weighed and put into a POLYPOT. Thisweighing is carried out in such a manner that, per 100 parts by weightof the ferrite powders, 20 to 40 parts (27 parts in the present example)by weight of the glutaric acid dimethyl, 2 to 4 parts (3 parts in thepresent example) by weight of the triacetin, and 1 to 5 parts (3 partsin the present example) by weight of the carboxylic copolymer aremeasured/taken. Further, zirconia balls are put into the POLYPOT, andwet mixing by the ball milling method is performed to obtain the ferriteslurry.

1.3: Mixing of Gelatinizing Agent

The thus obtained ferrite slurry, “4,4′-diphenylmethane diisocyanate andethylene glycol” serving as the gelatinizing agent, and“6-Dimethylamino-1-hexanol” serving as a catalyst for reaction areweighed as follows, and they are mixed by a mixer (a hybrid mixer). Thisweighing is carried out in such a manner that, per 100 parts by weightof the ferrite slurry, 1 to 10 parts (6.4 parts in the present example)by weight of the 4, 4′-diphenylmethane diisocyanate, 0.05 to 2.70 parts(0.35 parts in the present example) by weight of the ethylene glycol,and 0.03 to 2.00 parts (0.06 parts in the present example) by weight ofthe 6-Dimethylamino-1-hexanol are measured/taken. It should be notedthat an ion-exchange water is added before the mixing by an amount of0.01 to 2.70 parts (0.25 parts in the present example) by weight of theion-exchange water per 100 parts by weight of the ferrite slurry. Inthis manner, the slurry (ceramic slurry) which becomes material for themagnetic layer is obtained.

2. Manufacturing the Layered-Body-Before-Fired

Subsequently, the method for manufacturing the layered inductors 10shown in FIG. 1 is described with reference to FIGS. 12 to 15. It shouldbe noted that FIGS. 12 to 15 show the method to manufacture a singlelayered inductor 10, in which a single formed compact/body having apredetermined shape (pattern) is formed in each of the sheets, and thosesheets are layered to manufacture the single layered inductor 10, forconvenience of description. However, in actuality, a plurality (e.g.,1000 to 3000) of compacts having the same shape (pattern) are formed inthe sheets, those sheets are layered to form a single layered body, andthereafter, the single layered body is cut to manufacture a plurality ofthe layered inductor 10 simultaneously.

Hereinafter, “a first shaping mold (lower mold) and a second shapingmold (upper mold)” used to shape or form a sheet 4N (N is an integernumber between 1 to 8, that is, the sheet 4N is any one of sheets 41 to48) are expressed as “the first shaping mold 5N and the second shapingmold 6N”, respectively. That is, the highest-order digit of numeralgiven to the first shaping mold for each of the sheets is “5”, and thehighest-order digit of numeral given to the second shaping mold for eachof the sheets is “6”. The lowest-order digit of numeral given to “thespecific first shaping mold and the specific second shaping mold”coincides with the lowest-order digit of numeral given to a sheet whichis formed by “the specific first shaping mold and the specific secondshaping mold”. Accordingly, for example, shaping molds used for formingthe sheet 41 are the first shaping mold 51 and the second shaping mold61. Shaping molds used for forming the sheet 42 are the first shapingmold 52 and the second shaping mold 62.

2.1: Coating of Mold Release Agent

FIG. 12 shows an example to manufacture a single sheet 41 as arepresentative of the sheets 41 to 48. As shown in FIG. 12, the firstshaping mold 51 and the second shaping mold 61, each of which is astainless (e.g., aluminum alloy such as duralumin) plate-likerectangular parallelepiped, are prepared. Subsequently, mold releaseagents are coated on surfaces (planes) for molding of the first andsecond shaping molds 51, 61 so that a nonadherability film is formed oneach of the surfaces.

The film is formed in order to control/adjust a force (stress) in adirection of sheet thickness, the force being required to release acompact formed on the surface for molding from the surface (hereinafter,this force is referred to as “a mold release force”). It becomes harderto release the compact from the surface for molding, as the mold releaseforce becomes greater. In the present example, the mold release forcesfor the first shaping molds 51 to 58 are adjusted so as to be greaterthan the mold release forces for the second shaping molds 61 to 68,respectively. That is, for example, the mold release force for the firstshaping mold 51 is greater than the mold release force for the secondshaping mold 61.

Further, among the forces for the first shaping molds 51 to 58, the moldrelease force for the first shaping mold 51 is greater than any of themold release forces for the first shaping molds 52 to 58. Furthermore, aforce (hereinafter, referred to as “a sheet separating force”) in thedirection of sheet thickness required to pull apart (separate) one ofthe sheets from another sheet which are layered on and pressure bondedto the one is controlled (adjusted) to be greater than the mold releaseforce for the first shaping mold 51.

For the film, fluorine resin, silicon resin, fluorine oil, silicon oil,plating, and films by CVD and PVD etc., may be used. When the fluorineresin, the silicon resin, the fluorine oil, or the silicon oil is used,the film is formed by spraying or dipping. In such a case, the moldrelease force is controlled based on a kind of the resin, surfaceroughness, thickness of the film, etc. In the present example, a PETfilm is applied, or a film made from/of a mixture of the fluorine resin(manufactured by, for example, DAIKIN KOGYO, DIE-FREE 3130, 50 parts byweight) and isooctane (50 parts by weight) is formed.

2.2: Printing of a Conductive Pattern

Subsequently (or separately), a paste (hereinafter referred to as “aconductive paste”) which will later become the coil portion (conductivebody portion) 30 is prepared. As shown in (a) of FIG. 12, the conductivepaste are formed on “the surfaces for molding of the first shaping mold51 on which the film is formed” and on “the surfaces for molding of thesecond shaping mold 61 on which the film is formed”, according to screenprinting technique, a metallic mask technique, and so on.

At this time, the conductive paste is formed/shaped on the surface formolding of the first shaping mold 51 in such a manner that theconductive paste has a shape, which is roughly the same shape as thecompact which will become the main conductive layer 31 a included in thesheet 41 shown in FIG. 3, and which has a height lower than thethickness of the sheet 41. This compact is referred to as “a firstcompact”. In addition, the conductive paste is formed on the surface formolding of the second shaping mold 61 in such a manner that theconductive paste has a shape, which is roughly the same shape as thecompact which will become the via-connection portion 31 b included inthe sheet 41 shown in FIG. 3, and which has a height lower than thethickness of the sheet 41. This compact is referred to as “a secondcompact”. The thickness of the second compact is adjusted in such amanner that “a sum of the thickness of the first compact and thethickness of the second compact” is equal to or a little bit greaterthan the thickness of the sheet 41.

For the conductive paste, for example, “silver powders” as theconductive powders, “resins such as phenol resin, urethane resin,acrylic resin, butyral resin, ethyl cellulose, epoxy resin, andtheobromine, or resin precursors” as the resin component, and “mixturesof organic solvents such as butyl acetate carbitol, butyl carbitol,2-ethyl hexanol and terpineol” as the solvent, are used. The formedconductive paste (formed compact) is fixed/hardened throughpredetermined processes. For example, the paste including the phenolresins is fixed by heat.

It should be noted that, in the present example, the conductive pastecomprises 100 parts by weight of silver (Ag) powders, 4 to 10 (6.0 inthe present example) parts by weight of thermosetting phenol resin, 2 to8 (5.3 in the present example) parts by weight of butyl acetatecarbitol, and 2 to 10 (4.5 in the present example) parts by weight ofmelamine resin powders (grain diameter 2 μm). Here, an amount of themelamine resin powders is adjusted in such a manner that a difference inshrinkage between the magnetic body portion (ferrite) 20 and the coilportion (conductive portion) 30, when they are fired, becomes as smallas possible.

2.3: Setting Up of the Molds

Subsequently, as shown in (b) of FIG. 12, the second shaping mold 61 isplaced above the surface for molding of the first shaping mold 51 onwhich the first compact has been formed, with placing/interposing aspacer S having a height which is the same as the thickness of the sheet41 between the first shaping mold 51 and the second shaping mold 61. Atthis time, the second shaping mold 61 is placed in such a manner that“the surface for molding on which the second compact has been formed”opposes to (or face) “the surface for molding of the first shaping mold51 on which the first compact has been formed”. This allows the firstshaping mold 51 and the second shaping mold 61 to be arranged and placedin such a manner that “the surface (plane) for molding of the firstshaping mold 51 on which the first compact has been formed” and “thesurface (plane) for molding of the second shaping mold on which thesecond compact has been formed” oppose in parallel to each other, withan interspace/gap whose height is the same as the thickness of the sheet41, and in such a manner that a top surface of the first compact and atop surface of the second compact contact with each other. It should benoted that a space H defined and formed by “the first shaping mold 51,the second shaping mold 61, and the spacer S” has a profile which is thesame as a profile of the sheet 41 (i.e., the rectangularparallelepiped).

2.4: Pouring of the Slurry into the Molds and Fixing of the Slurry

Subsequently, as shown in (c) of FIG. 12, “the ceramic slurry which isthe material for the magnetic body portion 20” adjusted as describedabove is poured into the space H.

Subsequently, the ceramic slurry which was filled/poured into the spaceH is left for 10 to 30 hours (15 hours in the present example) so as tobe fixed/hardened (or solidified). As a result, the sheet 41 is obtainedin a state where the first shaping mold 51 and the second shaping mold61 are attached onto the lower surface and the upper surface of thesheet 41 (i.e., onto both end faces in the direction of the sheetthickness), respectively. Accordingly, the ceramic slurry isformed/shaped so as to have a shape which is the same as the shape (therectangular parallelepiped) of the sheet 41.

As described above, the ceramic slurry contains the ceramic powders, thedispersion media, and the gelatinizing agent. In addition, it containsthe dispersion aid and the catalyst, if necessary. The gelatinizingagent functions to fix/harden the ceramic powders. This fixation of theceramic slurry allows “the compact obtained by fixing the conductivepaste (i.e., the conductive-layer-before-fired) and theceramic-compact-before-fired (i.e., the magnetic-layer-before-fired)” tobe united (or integrated/consolidated). Further, the gelatinizing agentfunctions as a binder to bond the ceramic green sheets to one anotherwhen they are layered.

2.5: Releasing of the Molds

Subsequently, as shown in (d) of FIG. 12, only the second shaping mold61 is removed from the sheet 41 to which the first and second shapingmolds 51, 61 have been attached. As mentioned above, the mold releaseforce for the first shaping mold 51 is adjusted to be greater than themold release force for the second shaping mold 61. Accordingly, only thesecond shaping mold 61 can be easily released or removed by applying apulling force to the first and second shaping molds 51, 61 in thedirection of sheet thickness (in the vertical direction) so that thesemolds separate from each other. As a result, as shown in (d) of FIG. 12,the sheet 41, to which only the first shaping mold 51 is attached, isobtained.

FIG. 13 shows an example to manufacture a sheet 42. (a) to (d) of FIG.13 correspond to (a) to (d) of FIG. 12, respectively. The method formanufacturing the sheet 42 shown in (a) to (d) of FIG. 13 is similar tothe method for manufacturing the sheet 41, except the followings.

-   -   The conductive paste is formed on the surfaces for molding of        the first shaping mold 52 on which the film is formed in such a        manner that the conductive paste has a shape, which is roughly        the same shape as the compact which will become the        via-connection portion 32 b included in the sheet 42 shown in        FIG. 5, and which has a height lower than the thickness of the        sheet 42. This compact is referred to as “a third compact”.    -   The conductive paste is formed on the surfaces for molding of        the second shaping mold 62 on which the film is formed in such a        manner that the conductive paste has a shape, which is roughly        the same shape as the compact which will become the main        conductive layer 31 a included in the sheet 42 shown in FIG. 5,        and which has a height lower than the thickness of the sheet 42.        This compact is referred to as “a fourth compact”. It should be        noted that the thickness of the fourth compact is adjusted in        such a manner that “a sum of the thickness of the third compact        and the thickness of the fourth compact” is “equal to or a        little bit greater than “the thickness of the sheet 42”. In        actuality, the fourth compact is formed in such a manner that        the thickness of the fourth compact coincides with the thickness        of the first compact.

In this manner, as shown in (d) of FIG. 13, the sheet 42, to which onlythe first shaping mold 52 is attached, is obtained.

The sheets 43 to 48 can be obtained according to the same method formanufacturing as the methods for manufacturing the sheet 41 and thesheet 42, as described above.

2.6: Layering

Subsequently, as shown in (a) of FIG. 14, the sheet 42 to which thefirst shaping mold 52 is attached (refer to (d) of FIG. 13) is reversed(turned over) together with the first shaping mold 52, and is placed onthe plane (upper surface) of the sheet 41, the upper surface beingexposed by the removal of the second shaping mold 61. At this time,“dispersion media for increasing an adhesion between the sheets” isapplied onto the exposed surface of the sheet 41 and the exposed surfaceof the sheet 42. Accordingly, the exposed plane (surface) of the sheet41 and the exposed plane (surface) of the sheet 42 contact with eachother. Thereafter, the first shaping mold 52 is pressed toward the firstshaping mold 51 at a pressure of 50 kg/cm² or more. As a result, thesheet 41 and the sheet 42 are pressure bonded. At this stage, the sheet41 and the sheet 42 are sandwiched between the first shaping mold 51 andthe first shaping mold 52.

Subsequently, as shown in (b) of FIG. 14, only the first shaping mold 52is removed from the layered body of the sheets 41 and 42 to which thefirst shaping molds 51 and 52 have been attached. As mentioned above,the mold release force for the first shaping mold 51 is adjusted to begreater than the mold release force for the first shaping mold 52, andthe sheet separating force is adjusted to be greater than the moldrelease force for the first shaping mold 51. Accordingly, only the firstshaping mold 52 can be easily released or removed by applying a pullingforce to the first shaping molds 51, 52 in the direction of sheetthickness (in the vertical direction) to separate from each other. As aresult, as shown in (b) of FIG. 14, the layered body (the number oflayering=2) of the sheets 41 and 42, to which only the first shapingmold 51 is attached, is obtained.

Subsequently, the sheet 43 is manufactured according to processessimilar to the processes shown in FIGS. 12 and 13, and then is layeredonto and pressure bonded to the layered body of sheets 41 and 42. As aresult, a layered body (the number of layering=3) of sheets 41, 42, and43, to which only the first shaping mold 51 is attached, is obtained. Byrepeating such processes, a layered body of sheets 41 to 48, to whichonly the first shaping mold 51 is attached, is obtained, as shown in (a)of FIG. 15.

Thereafter, as shown in (b) of FIG. 15, the first shaping mold 51 isremoved from the layered body of the sheets 41 to 48 to which only thefirst shaping molds 51 have been attached. As mentioned above, the sheetseparating force is adjusted to be greater than the mold release forcefor the first shaping mold 51. Accordingly, the first shaping mold 51can be easily released or removed by applying a pulling force to thefirst shaping mold 51 in the direction of sheet thickness (in thevertical direction) so that the mold 51 is separate from the layeredbody of the sheets 41 to 48. In this manner, the layered body shown in(b) of FIG. 15 is obtained/manufactured.

Further, “an uppermost ceramic green sheet” consisting of the magneticlayer only without including the conductive layer is layered andpressure bonded onto an upper surface of the layered body, and “anlowermost ceramic green sheet” consisting of the magnetic layer onlywithout including the conductive layer is layered and pressure bondedonto a lower surface of the layered body. In this manner, alayered-inductor-before-fired for the layered inductor 10 shown in FIG.1 is obtained. It should be noted that “the lowermost ceramic greensheet” may be formed on the first shaping mold 51, and then, the layeredbody of the sheets 41 to 48 may be formed thereon according to themethod described above, and thereafter, “the uppermost ceramic greensheet” may be layered thereon.

Next will be described another method for manufacturing the layeredinductors 10 shown in FIG. 1 with reference to FIGS. 16 to 21. In this,method, each of the sheets 41 to 48 shown in FIGS. 3 to 11,respectively, is actually formed from two sheets. For example, the sheet41 is manufactured by layering and pressure bonding a sheet 41 a shownin FIG. 16 and a sheet 41 b shown in FIG. 17. The sheet 41 a is a sheetto form the main conducive layer 31 a. The sheet 41 b is a sheet to fromthe via-connection portion 31 b. Each of the other sheets 42 to 48consists of two sheets, i.e., one to form the conductive layer and theother one to form the via connection portion.

It should be noted that “a first shaping mold (lower mold) and a secondshaping mold (upper mold)” used for forming the sheet 41 a are expressedas “the first shaping mold 51 a and the second shaping mold 61 a”,respectively. Similarly, “a first shaping mold (lower mold) and a secondshaping mold (upper mold)” used for forming the sheet 41 b are expressedas “the first shaping mold 51 b and the second shaping mold 61 b”,respectively. That is, the highest-order digit of numeral given to thefirst shaping mold for each of the sheets is “5”, and the highest-orderdigit of numeral given to the second shaping mold for each of the sheetsis “6”. The last two digits of numeral given to “the first shaping moldand the second shaping mold” coincide with the last two digits ofnumeral given to a sheet which is formed by the molds.

FIG. 18 shows an example to manufacture a single sheet 41 a as arepresentative of the sheets 41 a to 48 a and sheets 41 b to 48 b. FIG.19 shows an example to manufacture a single sheet 41 b. The sheet 41 ismanufactured through processes shown in FIG. 20. Thereafter, the sheets42 a, 42 b, 43 a, 43 b, 44 a, 44 b, 45 a, 45 b, 46 a, 46 b, 47 a, 47 b,48 a, and 48 b are sequentially layered on the sheet 41 in this order.

Firstly, as shown in (a) of FIG. 18, the first shaping mold 51 a and thesecond shaping mold 61 a are prepared. The films described above areformed on the surfaces for molding of the shaping molds 51 a and 61 a byapplying the mold release agents to the surfaces. Such films are formedon the surfaces for molding of the other molds.

By means of the films, the mold release forces for the first shapingmolds 51 a to 58 a, and 51 b to 58 b are adjusted to be greater than themold release forces for the second shaping molds 61 a to 68 a, and 61 bto 68 b, respectively. That is, for example, the mold release force forthe first shaping mold 51 a is greater than the mold release force forthe second shaping mold 61 a.

Further, the mold release force for the first shaping mold 51 a isadjusted to be greater than any of the mold release forces for the firstshaping molds 52 a to 58 a and 51 b to 58 b. Furthermore, the sheetseparating force is adjusted to be greater than the mold release forcefor the first shaping mold 51 a.

Subsequently (or separately), the conductive paste described above isprepared. As shown in (a) of FIG. 18, the conductive paste is formed on“the surfaces for molding of the first shaping mold 51 a on which thefilm is formed” by a screen printing technique, etc. At this time, theconductive paste is formed in such a manner that the conductive pastehas a shape, which is roughly the same shape as the compact included inthe sheet 41 a shown in FIG. 16, and which has a height which is thesame as (or a little bit higher) than the thickness of the sheet 41 a.

Subsequently, as shown in (b) of FIG. 18, the second shaping mold 61 ais placed above the surface for molding of the first shaping mold 51 aon which the compact has been formed, in such a manner that the surfacefor molding of the second shaping mold 61 a on which the film has beenformed faces downward. At this time, a spacer S having a height which isthe same as the thickness of the sheet 41 a is interposed (placed)between the first shaping mold 51 a and the second shaping mold 61 a. Itshould be noted that a space H defined and formed by “the first shapingmold 51 a, the second shaping mold 61 a, and the spacer S” has a profilewhich is the same as a profile of the sheet 41 a (i.e., the rectangularparallelepiped).

Subsequently, as shown in (c) of FIG. 18, “the ceramic slurry which willbecome the material for the magnetic body portion 20” adjusted asdescribed above is poured into the space H.

Subsequently, the ceramic slurry which was filled into the space H isleft for 10 to 30 hours (15 hours in the present example) to be fixed(or solidified). As a result, the sheet 41 a is obtained in a statewhere the first shaping mold 51 a and the second shaping mold 61 a areattached onto the lower surface and the upper surface of the sheet 41(i.e., onto both end faces in the direction of the sheet thickness),respectively.

Subsequently, as shown in (d) of FIG. 18, only the second shaping mold61 a is removed from the sheet 41 a to which the first and secondshaping molds 51 a, 61 a have been attached. As mentioned above, themold release force for the first shaping mold 51 a is adjusted to begreater than the mold release force for the second shaping mold 61 a.Accordingly, only the second shaping mold 61 a can be easily released orremoved by applying a pulling force to the first and second shapingmolds 51 a, 61 a in the direction of sheet thickness (in the verticaldirection) so that the molds separate from each other. As a result, asshown in (d) of FIG. 18, the sheet 41 a, to which only the first shapingmold 51 a is attached, is obtained.

FIG. 19 shows an example to manufacture the sheet 41 b. (a) to (d) ofFIG. 19 correspond to (a) to (d) of FIG. 18, respectively. The methodfor manufacturing the sheet 41 b shown in (a) to (d) of FIG. 19 issimilar to the method for manufacturing the sheet 41 a shown in (a) to(d) of FIG. 18, except that a shape of the conductive paste which isformed on the surface for molding of the first shaping mold 51 b isdifferent from a shape of the conductive paste which is formed on thesurface for molding of the first shaping mold 51 a.

More specifically, the conductive paste is formed on the surfaces formolding of the first shaping mold 51 b in such a manner that theconductive paste has a shape, which is roughly the same shape as thecompact included in the sheet 41 b shown in FIG. 17, and which has aheight which is the same as (or a little bit higher than) the thicknessof the sheet 41 b. Detail descriptions on the method for manufacturingthe sheet 41 b shown in (a) to (d) of FIG. 19 are omitted. In thismanner, the sheet 41 b to which only the first shaping mold 51 b isattached is obtained, as shown in (d) of FIG. 19.

Subsequently, as shown in (a) of FIG. 20, the sheet 41 b to which thefirst shaping mold 51 b is attached (refer to (d) of FIG. 19) isreversed (turned over) together with the first shaping mold 51 b, and isplaced on the plane (upper surface) of the sheet 41 a, the upper surfacebeing exposed by the removal of the second shaping mold 61 a. At thistime, “dispersion media for increasing an adhesion between the sheets”is applied onto the exposed surface of the sheet 41 a and the exposedsurface of the sheet 41 b. Accordingly, the exposed plane (surface) ofthe sheet 41 a and the exposed plane (surface) of the sheet 41 b contactwith each other. Thereafter, the first shaping mold 51 b is pressedtoward the first shaping mold 51 a at a pressure of 50 kg/cm² or more.As a result, the sheet 41 a and the sheet 41 b are pressure bonded sothat the sheet 41 (refer to FIG. 3) having these sheets (41 a, 41 b) isobtained. At this stage, the sheet 41 is sandwiched between the firstshaping mold 51 a and the first shaping mold 51 b.

Subsequently, as shown in (b) of FIG. 20, only the first shaping mold 51b is removed from the layered body (i.e. the sheet 41) of the sheets 41a and 41 b to which the first shaping molds 51 a and 51 b have beenattached. As mentioned above, the mold release force for the firstshaping mold 51 a is adjusted to be greater than the mold release forcefor the first shaping mold 51 b, and the sheet separating force isadjusted to be greater than the mold release force for the first shapingmold 51 a. Accordingly, only the first shaping mold 51 b can be easilyreleased or removed by applying a pulling force to the first shapingmolds 51 a, 51 b in the direction of sheet thickness to separate fromeach other. As a result, as shown in (b) of FIG. 20, the layered body(the number of layering=2) of the sheets 41 a and 41 b, to which onlythe first shaping mold 51 a is attached, is obtained.

Subsequently, the sheet 42 a is manufactured according to processessimilar to the processes shown in FIG. 19. The sheet 42 a is thenlayered and pressure bonded onto the layered body of sheets 41 a and 41b according to processes similar to the processes shown in (a) and (b)of FIG. 20. As a result, a layered body (the number of layering=3) ofsheets 41 a, 41 b, and 42 a, to which only the first shaping mold 51 ais attached, is obtained. By repeating these processes, a layered bodyof sheets 41 to 48, to which only the first shaping mold 51 a isattached, is obtained, as shown in (a) of FIG. 21.

Thereafter, as shown in (b) of FIG. 21, the first shaping mold 51 a isremoved from the layered body of the sheets 41 to 48 to which only thefirst shaping molds 51 a have been attached. As mentioned above, thesheet separating force is adjusted to be greater than the mold releaseforce for the first shaping mold 51 a. Accordingly, the first shapingmold 51 a can be easily released or removed by applying a pulling forceto the first shaping mold 51 a in the direction of sheet thickness sothat the mold 51 is separate from the layered body of the sheets 41 to48. In this manner, the layered body shown in (b) of FIG. 21 ismanufactured.

Further, “an uppermost ceramic green sheet” consisting of the magneticlayer only without including the conductive layer is layered andpressure bonded onto an upper surface of the layered body, and “anlowermost ceramic green sheet” consisting of the magnetic layer onlywithout including the conductive layer is layered and pressure bondedonto a lower surface of the layered body. In this manner, alayered-inductor-before-fired for the layered inductor 10 shown in FIG.1 is obtained.

2.7: Drying

Subsequently, the thus obtained layered-body-before-fired is dried by adrying oven. After the drying process, the layered body is cut in such amanner that terminals T shown in FIGS. 3 and 11 are exposed at edges ofthe layered body (refer to cutting lines Cut shown in FIGS. 3 and 11).

2.8: Printing of Electrode Terminals

Subsequently, electrode terminals each of which has a predeterminedshape are formed by printing or dipping. It should be noted that theelectrode terminals may be printed after a firing process describedlater.

2.8: Firing (Burning/Sintering)

Subsequently, the layered-body-before-fired is fired.

The pattern of temperature change when firing (firing profile) is shownin Table 1 and by a solid line in FIG. 22.

TABLE 1 Time(h) 0 5 10 20 25 25.2 27.2 30.2 Temperature 0 200 200 500500 900 900 30 (° C.)

It should be noted that a conventional firing profile is shown in Table2 and by a dashed line in FIG. 22.

TABLE 2 Time(h) 0 5 10 20 25 30 32 35 Temperature 0 200 200 500 500 900900 30 (° C.)

It is clear from comparison between the Table 1 and Table 2 (comparisonbetween the solid line and the dashed line in FIG. 22), that in thefiring profile according to the present embodiment (hereinafter referredto as “the present firing profile”), the temperature is rapidlyincreased up to 900° C. to fire the layered-body-before-fired, after “adegreasing period in which the temperature is kept at 500° C. for 5hours” which is a period between 20 to 25 hours after a start of risingtemperature. That is, a rate of temperature increase after thedegreasing period in the comparative example is “80° C./h”, whereas arate of temperature increase after the degreasing period in the presentfiring profile is “2000° C./h”. It should be noted that the presentfiring profile is not limited to the profile shown in Table 1, but maybe a different firing profile in which the temperature is increased from500° C. to a keeping temperature which is equal to or higher than 850°C. and is lower than or equal to 950° C. within 30 minutes, after thedegreasing period in which the temperature is kept at 500° C.

This rapid temperature increase up to the firing temperature after thedegreasing period allows a timing from which the magnetic body portion(ferrite) 20 including ferrite as the magnetic substance starts to befired to substantially coincide with a timing from which the coilportion (the conductive portion) 30 including silver as the conductivesubstance starts to be fired. In other words, it is possible that themagnetic body portion 20 and the coil portion 30 start to shrink due tofiring substantially simultaneously. In addition, in the presentembodiment, the conductive paste which is the material for the coilportion is adjusted in such a manner that the difference in shrinkagebetween the magnetic body portion 20 and the coil portion 30 becomes assmall as possible when they are fired, as described above.

As a result, in the layered inductor 10 according to the presentembodiment, “the delamination” described above does not occur. Further,the problem that “the inductor does not have the desired electricalcharacteristic”, such as a significant decrease of the inductance, doesnot arise.

Thereafter, the electrode terminals are formed on the fired layered bodyso that the layered inductor 10 shown in FIG. 1 is manufactured.

STRUCTURAL FEATURES OF THE PRESENT EMBODIMENT

Next will be described structural features of the layered inductor 10according to the present embodiment.

1. Definitions of Parameters.

First, various parameters are defined. Hereinafter, “a conductive layer”means the main conductive layer 31 a which is the coil portion exceptthe via-connection portion 31 b. All of the parameters defined below arevalues about the fired (post-fired) conductive layer and the fired(post-fired) magnetic layer.

As shown in FIG. 23, the conductive layer has a cross sectional surface(i.e., cross sectional view of the coil), obtained by cutting theconductive layer along a plane perpendicular to a longitudinal directionof the conductive layer (that is, a plane along a width direction of theconductive layer), which has a substantial trapezoid shape having anupper base U1 and a lower base D1.

<Thickness t1 of the Fired Conductive Layer>

As shown in FIG. 23, a thickness t1 of the fired conductive layer is aheight of the trapezoid shape described above. That is, the thickness t1of the fired conductive layer is a distance between the upper base U1and the lower base D1.

<Thickness t2 of a Specific Portion of the Fired Magnetic Layer>

As shown in FIG. 23, a thickness t2 of a specific portion of the firedmagnetic layer is a distance between the upper base U1 of one conductivelayer and the lower base D2 of another conductive layer adjacent to theone conductive layer in the direction of layering. In other words, “thespecific portion of the fired magnetic layer” means a portion of themagnetic layer existing between two conductive layers that are adjacentto each other in the direction of layering.

<A Pitch of the Fired Conductive Layers>

A pitch of the fired conductive layers is a distance between twoconductive layers adjacent to each other in the direction of layeringand is equal to t1+t2.

<A Width L1 of the Fired Conductive Layer>

A width L1 of the fired conductive layer, as shown in FIG. 23, is alength of the lower base D1 (i.e., a distance between a point a1 and apoint a2, that are both end portions of the lower base D1).

<A Base Angle θ of the Fired Conductive Layer>

A base angle θ of the fired conductive layer is an angle of thetrapezoid shape at the both end portions of the lower base D1.

For example, as shown in (a) of FIG. 24, when the shape of the crosssectional surface of the coil portion is “a substantial trapezoid shapehaving R shapes (or arc shapes) at corners of both end portions of theupper base”, a fiat portion FLT of the upper base is specified. Thepoints at both end portions of the flat portion FLT are defined as apoint b1 and a point b2. In this case, the thickness t1 of theconductive layer is a distance between the lower base D1 and the flatportion FLT. Further, as shown in (b) of FIG. 24, the base angle θ ofthe conductive layer is an angle between a straight line SL1 and thelower base D1, the straight line SL1 is a line passing through “the endpoint b1” and “the end point a1 of the lower base D1 being closer to thepoint b1 than the point b2”. In other words, the base angle θ of theconductive layer is an angle between the lower base D1 and the straightline SL1, the straight line SL1 passing through “the end point b1” and“the end point a1 of the lower base D1, which is a vertex of an obliqueline portion extending from the point b1 and the lower base D1”.

For example, as shown in (a) of FIG. 25, when the shape of the crosssectional surface of the coil portion is “a substantial trapezoid shapehaving a top portion at each of the portions near the both end portionsof the upper base”, the top portion c1 and the other top portion c2 isspecified. Further, a portion between the top portion c1 and the topportion c2 is defined as the flat portion FLT. In this case, thethickness t1 of the conductive layer is an average value of a distancebetween each of points on the upper base U1 within the thus defined flatportion FLT and the lower base D1 (refer to a straight line S1). Thethickness t2 of the specific portion of the magnetic layer is a distancebetween the flat potion FLT (shown by the straight line S1) of one ofthe conductive layers and the lower base D2 of the other conductivelayer adjacent to the one conductive layer in the direction of layering.In addition, as shown in (b) of FIG. 25, the base angle θ of theconductive layer is an angle between a straight line SL2 and the lowerbase D1, the straight line SL2 passing through “a point b at which thestraight line S1 intersects with the oblique line portion” and “an endpoint a1 of the lower base D1 which is a vertex of the oblique lineportion passing through the point b and the lower base D1”.

For example, as shown in (a) of FIG. 26, when the shape of the crosssectional surface of the coil portion is “a substantial trapezoid shapehaving R shapes (or arc shapes) at corners of both end portions of thelower base D1”, a point a1 and a point a2 of the end portions of thelower base D1 is defined as both end points of a flat portion of thelower base. In other words, the width L1 of the conductive layer is, asshown in (a) of FIG. 26, a length of the flat portion of the lower baseD1 (i.e., a distance between the point a1 and the point a2).Furthermore, as shown in (b) of FIG. 26, the base angle θ is an anglebetween a straight line SL3 and the lower base D1, the straight line SL3passing through the point a1 (or the point a2) and the point b (or thepoint b1) that are specified as described above.

As described above, the conductive layer of the inductor according tothe present invention has a cross-sectional shape having the substantialtrapezoid shape which has the upper base U1 and the lower base D1. Tothe contrary, in a comparative example as shown in (a) of FIG. 27 (i.e.the comparative example to which the present invention is not applied),when the shape of the cross sectional surface of the coil portion is notthe substantial trapezoid shape, but is “a substantial semi circularshape (an arc shape)”, a top portion d may be specified. In this case,the thickness t1 of the conductive layer is defined as a distancebetween the top portion d and the lower base D1. The thickness t2 of thespecific portion of the magnetic layer is a distance between the topportion d (see a straight line S2) of one conductive layer and the lowerbase D2 of another conductive layer adjacent to the one conductive layerin the direction of layering. Furthermore, as shown in (b) of FIG. 27,the base angle θ of the conductive layer in this comparative example isdefined as an angle between a tangent line SL4 to the oblique lineportion at a point a1 and the lower base D1, the point a1 being one ofboth end portions of the lower base D1.

2. The Structural Features

The layered inductors 10 of the embodiments according to the presentinvention have the features described below.

(Feature A)

A cross-sectional shape of the conductive layer has a substantialtrapezoid shape, and the base angle θ is equal to or greater than 50°and is smaller than or equal to 80°.

(Feature B)

The magnetic layers comprise “gaps (vertical direction gaps CK),extending so as to have a component along (or in) the direction oflayering of the layered body (in the direction of the thickness of theconductive layer) and connects/communicates between two conductivelayers adjacent to each other in the direction of layering”.

(Feature C)

The vertical direction gap CK extends downwardly, in a cross sectionalview of the coil portion (i.e., in a cross sectional view of theconductive layer and the magnetic layer, cut by a plane perpendicular tothe longitudinal direction of the conductive layer), so as to have “thecomponent along (or in) the direction of layering of the layered body”,the gap extending from “a surface of the conductive layer within ±30 μm(refer to a heavy line F1 shown in FIG. 28) along the surface of theconductive layer from one of end portions (e.g., points a1 shown inFIGS. 23 to 26) of the lower base D1 of the conductive layer”. Further,the vertical direction gap CK extends upwardly so as to have “thecomponent along (or in) the direction of layering of the layered body”from “a surface of the conductive layer “within ±30 μm (refer to a heavyline F2 shown in FIG. 28) along the surface of the conductive layer fromone of end portions (e.g., the point b or the point b1 shown in FIGS. 23to 26) of the upper base U1 of the conductive layer”.

(Feature D)

The conductive layer has a lot of (a great number of) holes/poresinside, and “a ratio of (total) area of the pores to the area of theconductive layer” in the cross sectional view of the coil portion (i.e.,in the cross sectional view of the conductive layer, cut by a planeperpendicular to the longitudinal direction of the conductive layer), isequal to or greater than 2% and is smaller than or equal to than 30%.

(Feature E)

An average of diameters of the pores is equal to or greater than 0.01·t1and is smaller than or equal to 0.20·t1. That is, the average diameter Dsatisfies an expression (1) described below.

0.01≦D/t1≦0.20  (1)

(Feature F)

The specific portion of the magnetic layer (the portion of the magneticlayer existing between two conductive layers that are adjacent to eachother in the direction of layering) has a relative density which isequal to or greater than 84% and is smaller than or equal to 92%. Itshould be noted that the relative density is a density of the magneticlayer, the density being 100% when there is no pores in the magneticlayer.

Further, the conductive layer has features as follows.

(Feature G)

The thickness t1 of the conductive layer and the thickness t2 of thespecific portion of the magnetic layer satisfy an expression (2)described below.

0.1≦t2/t1≦0.9  (2)

(Feature H)

The length L1 of the lower base is equal to or greater than 200 μm.

EXAMPLES OF THE PRESENT INVENTION AND COMPARATIVE EXAMPLES

Next will be described various examples (examples 1, 2, and 3) of thelayered inductor 10 according to the present invention with comparingthem with various comparative examples (comparative examples 1 and 2).It should be noted that the example 1 includes examples 1-1 to 1-5. Thecomparative example 1 includes comparative examples 1-1 to 1-3. Theexample 2 includes examples 2-1 to 2-5. The example 3 includes examples3-1 to 3-7. The comparative example 2 includes comparative examples 2-1to 2-6.

Common features between the examples 1, 2, and 3 and the comparativeexamples 1 and 2 are as follows.

-   -   The compositions of ferrite contained in the magnetic layer are        Fe₂O₃ (47.5 mol %)·NiO (16.3 mol %)·ZnO (27.3 mol %)·CuO (8.7        mol %)·MnO₂ (0.2 mol %).    -   A pattern of the coil portion 30 is as shown in FIG. 1. A number        of turns is 7.25.    -   A volume ratio of additive which is the melamine resin in the        conductive paste to silver (Ag) is 31.5%.

Conditions for firing of the examples 1, 2, and 3 and the comparativeexamples 1 and 2 are as follows.

-   -   The example 1: Keep 900° C. for 5 hours. The rate of temperature        increase after the degreasing period is shown in Table 1. That        is, the temperature is increased rapidly.    -   The comparative Example 1: Keep 900° C. for 2 hours. The rate of        temperature increase after the degreasing period is shown in        Table 1. That is, the temperature is increased rapidly.    -   The example 2: Keep 900° C. for 0.5 to 2 hours. The rate of        temperature increase after the degreasing period is shown in        Table 1. That is, the temperature is increased rapidly.    -   The example 3: Keep 850 to 900° C. for 0.5 to 5 hours. The rate        of temperature increase after the degreasing period is shown in        Table 1. That is, the temperature is increased rapidly.    -   The comparative Example 2: Keep 850 to 920° C. for 0.5 to 5        hours. The rate of temperature increase after the degreasing        period is shown in Table 1. That is, the temperature is        increased rapidly. However, the kinds, amounts, and grain        diameter of resin powders added to the conductive paste are        different from those in the conductive paste according to the        present examples.

Measurement results of various values about the thus manufacturedlayered inductors are shown in Table 3 to Table 7. It should be notedthat a percent defective in these Tables is a ratio of the number ofinductors whose inductance is equal to or smaller than 0.6 pH to thetotal number of the inductors. Each of the values in these Tables foreach of samples (e.g., the embodiment 1-1) is an average of the valuesfor each of the samples excluding defective samples, among the 30layered inductors (samples) that are simultaneously manufactured. Itshould also be noted that the example 1 whose data are shown in Table 3is the layered inductors having neither the vertical direction gaps northe side direction gaps (i.e., these gaps are not observed). The example2 whose data are shown in Table 5 is the layered inductors having thevertical direction gaps but having no side direction gaps.

TABLE 3 pitch base DC percent Sample t1 t2 ratio t1 + t2 L1 angle θinductance resistance defective Example 1 (μm) (μm) t2/t1 (μm) (μm) (°)(μH) (Ω) (%) Example 1-1 51 9 0.18 60 281 52 2.5 0.056 33 Example 1-2 3913 0.33 52 279 78 2.4 0.073 26 Example 1-3 35 15 0.43 50 285 73 2.90.081 17 Example 1-4 50 30 0.60 80 208 65 4.8 0.069 0 Example 1-5 45 280.62 73 285 67 3.3 0.063 0 t1: thickness of the coil t2: thickness ofthe magnetic layer L1: widh of the coil

TABLE 4 Sample pitch base DC percent Comparative t1 t2 ratio t1 + t2 L1angle θ inductance resistance defective Example 1 (μm) (μm) t2/t1 (μm)(μm) (°) (μH) (Ω) (%) Com. Ex. 1-1 60 5 0.08 65 281 3 2.9 0.047 97 Com.Ex. 1-2 53 15 0.28 68 276 38 3.1 0.053 53 Com. Ex. 1-3 42 29 0.69 71 21042 4.3 0.082 20 t1: thickness of the coil t2: thickness of the magneticlayer L1: widh of the coil

TABLE 5 pitch base DC percent Sample t1 t2 ratio t1 + t2 L1 angle θinductance resistance defective Example 2 (μm) (μm) t2/t1 (μm) (μm) (°)(μH) (Ω) (%) Example 2-1 48 9 0.19 57 279 55 2.3 0.059 20 Example 2-2 4720 0.43 67 280 68 3.0 0.060 3 Example 2-3 47 28 0.60 75 283 72 3.2 0.0580 Example 2-4 41 32 0.78 73 282 63 3.3 0.062 0 Example 2-5 41 32 0.78 73283 69 3.3 0.063 0 t1: thickness of the coil t2: thickness of themagnetic layer L1: widh of the coil

FIG. 29 is a graph showing a relationship between “the ratio t2/t1” and“the percent defective” shown in Table 3 to Table 5. It is clear fromFIG. 29 that the percent defective of “the examples 1 and 2 (refer to anellipse shown by a dashed line) of the present invention, each havingthe conductive layer whose cross sectional shape is the substantialtrapezoid shape, having an upper base and a lower base substantially andhaving the base angle θ which is equal to or greater than 50° and issmaller than or equal to 80°” is extremely lower than the percentdefective of “the comparative example 1 (refer to an ellipse shown by analternate long and short dash line) having the conductive layer whosecross sectional shape is the semi circular shape, but is not thetrapezoid shape”. In other words, the percent defective at “the specificrate t2/t1” of the example 1 or the example 2 is much lower than that ofthe comparative example 1. Accordingly, the inventors found that it ispossible to greatly decrease the percent defective, if “the crosssectional shape of the conductive layer is the substantial trapezoidshape, having an upper base and a lower base substantially and havingthe base angle θ which is equal to or greater than 50° and is smallerthan or equal to 80° (preferably the base angle θ being equal to orgreater than 52° and smaller than or equal to 78° (refer to the featureA described above). It should be noted that this condition is referredto as “a condition A”.

FIG. 30 is a photograph showing a vertical cross section of the layeredinductor according to the example 1-5 whose data are shown in Table 3.FIG. 31 is a magnified photograph of the vertical cross section of aportion around the coil portion shown in FIG. 30. It is understood fromthese photographs that there is no gap between the magnetic layer andthe conductive layer and there is no structural defect such as a gap(side direction gap) along the direction parallel to the layers (layerplanes).

FIG. 32 is a photograph showing a vertical cross section of the layeredinductor of the comparative example 1-3 whose data are shown in Table 4.It is understood from FIG. 32 that the base angle θ of the comparativeexample 1-3 is 42° which is small, and the shape of the cross section ofthe coil is not trapezoid (the shape is the substantial semi circularshape). As just described above, if the cross section of the coil is nottrapezoid substantially and the base angle θ is smaller than 50°, thegaps occur not only in the vertical direction (in the direction oflayering) but also in the side (horizontal) direction (in the directionparallel to the plane of the layer). As a result, the electricalcharacteristics of the layered inductor become unstable. Further, theside direction gap may cause the delamination and cause a problem in itsreliability.

FIG. 33 is a photograph showing a vertical cross section of the layeredinductor according to the example 2-3 whose data are shown in Table 5.FIG. 34 is a magnified photograph of the vertical cross section of aportion around the coil portion shown in FIG. 33. It is understood fromthese photographs that the above described vertical direction gaps occurat outer circumference side of the coil portion. It is inferred thatthis happens because stress concentrates on the end portions of thelower base and the end portions of the upper base by having the shape ofthe vertical cross section of the coil be substantially trapezoid, andtherefore, the vertical direction gaps are formed from those endportions. The vertical direction gaps do not cause the delamination,unlike the side direction gaps, and can release the great internalstress added to the magnetic layer (ferrite). The great internal stressadded to the magnetic layer (ferrite) causes a big change in theinductance. Consequently, having such vertical direction gaps occurpositively can allow the inductance to be a value close to the desiredand targeted value stably.

In view of the above, the inventors found that it is possible to obtainlayered inductors whose electrical characteristics such as theinductance are more stable, if the conditions B and C described beloware also satisfied.

(Condition B)

The magnetic layer comprises the vertical direction gaps CK (refer tothe above Feature B). It is preferable that the vertical direction gapCK extend so as to connect/communicate between two conductive layersadjacent to each other in the direction of layering.

(Condition C)

It is preferable that the vertical direction gap CK extend downwardly,in a cross sectional view of the coil, so as to have “the componentalong (or in) the direction of layering” from “the surface of theconductive layer within ±30 μm (refer to the heavy line F1 shown in FIG.28) along the surface of the conductive layer from the end portions ofthe lower base D1 of the conductive layer” (refer to the above FeatureC). Further, It is preferable that the vertical direction gap CK extendupwardly, in a cross sectional view of the coil, so as to have “thecomponent along (or in) the direction of layering” from “the surface ofthe conductive layer within ±30 μm (refer to the heavy line F2 shown inFIG. 28) along the surface of the conductive layer from end portions ofthe upper base U1 of the conductive layer” (refer to the above FeatureC).

TABLE 6 Area Average ratio diameter relative pitch base of of thedensity of Sample t1 t2 ratio t1 + t2 L1 angle θ holes holes ratioferrite A B C Example 3 (μm) (μm) t2/t1 (μm) (μm) (°) (%) D(μm) D/t1 (%)(μH) (Ω) (%) Example 3-1 49 31 0.63 80 284 70 2.9 0.6 0.01 84 3.5 0.0570 Example 3-2 41 32 0.78 73 283 69 4.5 2.6 0.06 86 3.3 0.063 0 Example3-3 45 28 0.62 73 285 67 8.7 1.5 0.03 92 3.3 0.063 0 Example 3-4 47 280.60 75 283 72 5.9 2.6 0.06 88 3.2 0.058 0 Example 3-5 41 32 0.78 73 28263 18.8 5.0 0.12 89 3.3 0.062 0 Example 3-6 50 30 0.60 80 208 65 22.18.6 0.19 91 4.8 0.069 0 Example 3-7 45 30 0.67 75 281 64 28.3 7.9 0.1888 3.2 0.078 0 t1: thickness of the coil t2: thickness of the magneticlayer L1: widh of the coil A: Inductance B: DC resistance C: percentdefective

Among the samples shown in Table 6, none of the vertical direction gapis observed in any of the examples 3-1, 3-3, and 3-6, and the verticaldirection gaps are observed in the examples 3-2, 3-4, 3-5, and 3-7. Noneof the side direction gaps is observed in any of the examples 3-1 to3-7.

TABLE 7 Area Average ratio diameter relative Sample pitch base of of thedensity of Comparative t1 t2 ratio t1 + t2 L1 angle θ holes holes ratioferrite A B C Example 2 (μm) (μm) t2/t1 (μm) (μm) (°) (%) D(μm) D/t1 (%)(μH) (Ω) (%) Com. Ex. 2-1 41 30 0.73 71 280 64 1.2 1.6 0.04 92 2.9 0.06473 Com. Ex. 2-2 43 32 0.74 75 281 72 35.4 13.9 0.19 89 3.1 0.111 0 Com.Ex. 2-3 47 31 0.66 78 282 68 3.2 0.4 0.009 91 2.7 0.066 63 Com. Ex. 2-446 32 0.70 78 278 63 8.7 9.7 0.21 92 3.2 0.063 37 Com. Ex. 2-5 48 300.63 78 280 79 2.8 1.3 0.03 80 3.8 0.055 0 Com. Ex. 2-6 49 30 0.61 79283 66 7.8 4.9 0.10 94 3.1 0.063 0 t1: thickness of the coil t2:thickness of the magnetic layer L1: widh of the coil A: Inductance B: DCresistance C: percent defective

FIG. 35 is a photograph showing a vertical cross section of the layeredinductor according to the example 3-3, whose data are shown in Table 6.In this photograph, a light gray portion shows the coil portion, and adark gray portion shows the magnetic layers. When focusing on the coilportion, circular pores are observed. An average diameter of the poresis the average diameter D of the pores shown in Tables 6 and 7. The arearatio of the pores shown in Tables 6 and 7 is a ratio of an area of thepores to a cross-sectional area of the coil, in the cross-sectional viewof the coil.

Findings are obtained from the samples whose data are shown in Table 7as follows.

The comparative Example 2-1: There are little pores and the delaminationoccurred.

The comparative Example 2-2: The side direction gap is not observed,however, the DC resistance is extremely large compared to the example 3.

The comparative Example 2-3: There are some pores, however, the averagediameter (D) of the pores is too small, and the delamination thereforeoccurred.

The comparative Example 2-4: Since the average diameter (D) of the poresis too large, the position of the gaps could not be controlled, andrandom gaps are therefore observed.

The comparative Example 2-5: The gaps are not observed, however, thereliability is low, because the inductance varied more than 20% in areliability test (a high-temperature and loading test: the inductor isoperated at 80° C. for 500 hours with 2 A, and a high-humidity andloading test: the inductor is operated at 40° C., at humidity 95%, for500 hours with 2 A).

The comparative Example 2-6: The gaps extend up to outer surfaces of theinductor, and the reliability is low, because the inductance varied morethan 20% in a reliability test (a high-temperature and loading test: theinductor is operated at 80° C. for 500 hours with 2 A, and ahigh-humidity and loading test: the inductor is operated at 40° C., athumidity 95%, for 500 hours with 2 A).

Based on a comparison between Table 6 and Table 7, it turned out thatthe percent defective of the any of the samples in the example 3 is“0%”, however, there are samples in the comparative example 2 which havehigh percent defective. Further, there is no desirable inductor in thecomparative example 2 as the findings described above. Accordingly, theinventors found that it is possible to obtain layered inductors withextremely high reliability, if the conditions D and E described beloware satisfied, based on the careful comparison of values shown in Table6 to Table 7.

(Condition D)

It is preferable that “the ratio (the area ratio of the pores) of thearea of the pores to the area of the conductive layer” in thecross-sectional view of the coil be equal to or greater than 2% and besmaller than or equal to 30% (refer to the above Feature D). It is morepreferable that “the ratio of the area of the pores to the area of theconductive layer” be equal to or greater than 2.9% and be smaller thanor equal to 28.3%. This is because, if the area ratio of the pores issmaller than 2%, a hardness of the coil portion is so high that thestress can not be concentrated on the end portions of the upper base andthe lower base of the coil portion, and accordingly, the large sidedirection gaps occur in the magnetic layers, and thereby, the inductancecan not be a value in proximity to the desired and targeted value. Onthe other hand, if the area ratio of the pores is greater than 30%, thecross-sectional area of the coil portion is excessively small, andaccordingly, the resistance of the coil portion becomes excessivelylarge.

(Condition E)

It is preferable that the average diameter D of the pores be equal to orgreater than 0.0141 and be smaller than or equal to 0.20·t1. In otherwords, it is preferable that the ratio D/t1 be equal to or greater than0.01 and be smaller than or equal to 0.20 (refer to the above FeatureE). It is more preferable that the average diameter D of the pores ofthe conductive layers be equal to or greater than 0.01·t1 and be smallerthan or equal to 0.19·t1. This is because the stress can be concentratemore easily on the end portions of the upper base and the lower base ofthe coil portion, when the relatively small pores satisfying the abovecondition are dispersed in the coil portion.

(Condition F)

It is preferable that the relative density of the specific portion ofthe magnetic layer (the portion of the magnetic layer existing betweentwo conductive layers that are adjacent to each other in the directionof layering) be equal to or greater than 84% and be smaller than orequal to 92% (refer to the above Feature F). In other words, it ispreferable that a porosity in the specific portion of the magnetic layerbe equal to or greater than 8% and be smaller than or equal to 16%. Thisis because, if the relative density is smaller than 84%, ahygroscopicity of the magnetic layer is so high that the reliability ofthe layered inductor becomes low. On the other hand, if the relativedensity is greater than 92%, uncontrollable side direction gaps occur inthe magnetic layers.

It should be noted that the example 3 (the examples 3-1 to 3-7)satisfies all of the conditions D, E, and F. To the contrary, thecomparative example 2 (the comparative examples 2-1 to 2-6) does notsatisfy at least one of the conditions D, E, and F.

Further, two groups of conditions consisting of “the condition A” and“the conditions B and C” may preferably be satisfied simultaneously,however, it is sufficient that the only “the conditions A” is satisfied.In addition, it is preferable that “the conditions D and E” among “theconditions D, E, and F” be simultaneously satisfied, and is morepreferable that those three conditions D, E, and F be simultaneouslysatisfied. When those conditions or the groups of conditions aresatisfied as described above, the layered inductor, having littleproblems due to the structural defects etc., compared to theconventional layered inductor, is provided.

Furthermore, it is understood from Tables 3 to 5, that conditions G andH described below are preferably satisfied.

(Condition G)

A ratio t2/t1 is equal to or greater than 0.1 and is smaller than orequal to 0.9 (refer to the above formula (2) of the Feature G). It ismore preferable that the ratio t2/t1 be equal to or greater than 0.18and equal to or smaller than 0.78.

More specifically, when the ratio t2/t1 is equal to or greater than 0.57(preferably 0.60), the percent defective is “0” in both the example 1and the example 2. That is, if the ratio t2/t1 is equal to or greaterthan 0.57 (preferably 0.60), no defective inductor is manufactured aslong as the cross-sectional shape has the substantial trapezoid shapedescribed above, irrespective of existence or nonexistence of thevertical direction gap (regardless of whether or not the inductor hasthe vertical direction gap). Further, it is understood from FIG. 29,that the percent defective of the example 2 having the verticaldirection gap (gaps) is lower than the percent defective of the example1 having no vertical direction gap, when the ratio t2/t1 is smaller than0.57 (preferably 0.60). In other words, when the ratio t2/t1 is smallerthan 0.57 (preferably 0.60), the vertical gap(s) can decrease thepercent defective.

(Condition H)

It is preferable that the length L1 of the lower base be equal to orgreater than 200 μm (refer to the above Feature H). It is morepreferable that the length L1 of the lower base be equal to or greaterthan 208 μm.

As described above, the embodiments of the present invention can providethe layered inductor, which has no structural defect, such as thedelamination, adversely affects the electrical characteristics of theinductor, and which can decrease the resistance of the coil portion byincreasing the cross-sectional are of the coil portion. It should benoted that the present invention is not limited to the aboveembodiments, but may be modified as appropriate without departing fromthe scope of the invention. For example, a pattern of turn of the coilmay be circular in plan view, or the number of turns of the coil portionis other than 7.25.

1. A layered inductor which is a layered body in which silver-basedconductive-layers-before-fired and ferrite-basedmagnetic-layers-before-fired are layered and simultaneously fired, andin which the conductive layers are via-connected so as to form a helicalcoil, wherein, a shape of a cross sectional surface of each of saidconductive layers cut by a plane perpendicular to a longitudinaldirection of each of said conductive layers is a substantial trapezoidshape having an upper base and a lower base; and a base angle θ of saidtrapezoid shape at both end portions of said lower base is equal to orgreater than 50° and is smaller than or equal to 80°.
 2. A layeredinductor according to claim 1, wherein one of said magnetic layers has agap extending so as to have a component along a direction of layering ofsaid layered body and so as to connect between two of said conductivelayers adjacent to each other in the direction of layering.
 3. A layeredinductor according to claim 2, wherein said gap, in a cross sectionalview of said conductive layers and said magnetic layers, cut by a planeperpendicular to the longitudinal direction of the conductive layers,extends downwardly so as to have said component along said direction oflayering of said layered body, from a surface of one of said conductivelayers within ±30 μm along the surface of said one of the conductivelayers from one of end portions of said lower base of said one of theconductive layers, and extends upwardly so as to have said componentalong the direction of layering, from a surface of one of saidconductive layers within ±30 μm along the surface of said one of theconductive layers from one of end portions of said upper base of saidone of the conductive layers.
 4. A layered inductor according to claim1, wherein said conductive layers has a great number of pores, and aratio of a total area of the pores to an area of said conductive layerin a cross sectional view of said conductive layers, cut by a planeperpendicular to said longitudinal direction of said conductive layers,is equal to or greater than 2% and is smaller than or equal to 30%; anda ratio D/t1 of an average diameter D of said pores to a thickness t1 ofeach of said conductive layers that are fired is equal to or greaterthan 0.01 and is smaller than or equal to 0.20.
 5. A layered inductoraccording to claim 4, wherein a relative density of a portion of saidmagnetic layers that are fired, said portion existing between two ofsaid conductive layers that are adjacent to each other in said directionof layering, is equal to or greater than 84% and is smaller than orequal to 92%, wherein said relative density is 100% when it is assumedthat there is no pore in said magnetic layers.
 6. A layered inductoraccording to claim 2, wherein said conductive layers has a great numberof pores, and a ratio of a total area of the pores to an area of saidconductive layer in a cross sectional view of said conductive layers,cut by a plane perpendicular to said longitudinal direction of saidconductive layers, is equal to or greater than 2% and is smaller than orequal to 30%; and a ratio D/t1 of an average diameter D of said pores toa thickness t1 of each of said conductive layers that are fired is equalto or greater than 0.01 and is smaller than or equal to 0.20.
 7. Alayered inductor according to claim 3, wherein said conductive layershas a great number of pores, and a ratio of a total area of the pores toan area of said conductive layer in a cross sectional view of saidconductive layers, cut by a plane perpendicular to said longitudinaldirection of said conductive layers, is equal to or greater than 2% andis smaller than or equal to 30%; and a ratio D/t1 of an average diameterD of said pores to a thickness t1 of each of said conductive layers thatare fired is equal to or greater than 0.01 and is smaller than or equalto 0.20.